1. Field of the Invention
The present invention relates to a reflection mirror having a high reflectance (hereinbelow, referred to as a high reflectance mirror) used mainly for a reflection mirror in a rear-projection television or a backlight module for a small-sized liquid crystal display for a cellular phone etc.
2. Discussion of Background
As reflection mirrors used for electronic appliances such as flat panel displays etc., there have widely been used mirrors in which a metal film is utilized for reflection. For purposes of improving the luminance of an electronic appliance and energy saving, it is important to increase the reflectance of the reflection mirror. For instance, in a liquid crystal display used for a cellular phone etc., a mirror for reflecting the backlight is used. In such mirror, a film is used as the substrate to reduce the weight, and on the other hand, there is a demand of providing the reflection mirror having a high reflectance. Further, a plurality of reflection mirrors are needed in an optical system by which an image can be reflected on a large-sized screen of, for instance, a projection television. In such case, the light intensity decreases as the number of times of reflection increases. As a result, there causes the problem that the finally obtainable light intensity decreases so that the luminance of the screen becomes low. Accordingly, a reflection mirror having a higher reflectance than conventional reflection mirror is required.
As the material for the metal film, aluminum has conventionally been used. However, when the aluminum is used as the material for the metal film, there is the problem that the reflectance varies depending on an incident angle of light thereby causing fluctuation of the reflected color.
In order to solve the above-mentioned problem, there is a proposal that silver, having a higher reflectance in the visible-light region than aluminum, is used as the material for the metal film. However, although the silver has a high reflectance in the visible-light region in comparison with aluminum, the silver has drawbacks that durability to a moisture resistance, a saltwater resistance etc. is low; it is easily damaged because the strength of the film is low, and the adhesion of it to the substrate is poor.
As a mirror using an Ag film as the metal film, having a high reflectance and being excellent in durability, a high reflectance mirror formed by depositing on a glass substrate an Al2O3 film, an Ag film, an Al2O3 film and a TiO2 film in this order, is disclosed (see for example, JP-A-2003-4919). In this high reflectance mirror, oxygen is introduced when the Al2O3 film is formed on the Ag film surface on the opposite side of the substrate. Accordingly, there is the problem that the silver is easily oxidized and then to reduce the reflectance.
Further, in order to improve the adhesion between an Ag film and the substrate, a reflecting film in which a metal such as Ce or Nd is mixed in Ag is disclosed (see, for example, JP-A-2002-226927). However, this reflecting film is a single film of silver, and therefore, there is only description about the adhesion between the Ag film and the substrate, and there is no evaluation about the adhesion between the Ag film and the other layer at all.
Further, a reflection mirror wherein an Al2O3 film, a ZrO2 film and a SiO2 film are formed on an Ag film is disclosed (see, for example, JP-A-5-127004). This publication describes that the Al2O3 film is a protective film to improve the durability of the Ag film, the ZrO2 film is a film to improve the reflection efficiency and the SiO2 film is a protective film.
Further, the formation of a film of chromium oxide between the substrate and an Ag film in order to improve the adhesion between the substrate and the Ag film, is disclosed (see, for example, JP-A-2000-81505).
Further, there is description that an Al2O3 film is formed on an Ag film, and a layer of e.g. zirconium oxide, silicon dioxide, titanium oxide, hafnium oxide, tin oxide, antimony oxide, tungsten oxide or the like is formed to improve the durability (see, for example, JP-A-2000-241612).
Further, the formation of an undercoat film of silicon oxide between the substrate and an Ag film to improve the durability, is disclosed (see, for example, JP-A-2001-74922).
However, these reflecting films have the problem that the reflectance in a visible-light region is low.
It is an object of the present invention to provide a high reflectance mirror having a high reflectance in a visible-light region, being excellent in durability such as a moisture resistance, a saltwater resistance, etc. and having small dependence on incident angle (i.e., the fluctuation of the reflectance depending on an incident angle of light is little).